EP0663708B1 - Controlled optical amplifier - Google Patents

Controlled optical amplifier Download PDF

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Publication number
EP0663708B1
EP0663708B1 EP95400052A EP95400052A EP0663708B1 EP 0663708 B1 EP0663708 B1 EP 0663708B1 EP 95400052 A EP95400052 A EP 95400052A EP 95400052 A EP95400052 A EP 95400052A EP 0663708 B1 EP0663708 B1 EP 0663708B1
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EP
European Patent Office
Prior art keywords
amplifier
light
regulation
amplified
waveguide
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Expired - Lifetime
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EP95400052A
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German (de)
French (fr)
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EP0663708A1 (en
Inventor
Dominique Bayart
Bertrand Clesca
José Chesnoy
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Alcatel Lucent SAS
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Alcatel CIT SA
Alcatel SA
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
    • H01S3/1001—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by controlling the optical pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S2301/00—Functional characteristics
    • H01S2301/06—Gain non-linearity, distortion; Compensation thereof
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0064—Anti-reflection devices, e.g. optical isolaters
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06754—Fibre amplifiers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094003—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre
    • H01S3/094011—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre with bidirectional pumping, i.e. with injection of the pump light from both two ends of the fibre
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
    • H01S3/10015—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by monitoring or controlling, e.g. attenuating, the input signal

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optics & Photonics (AREA)
  • Optical Communication System (AREA)
  • Lasers (AREA)
  • Semiconductor Lasers (AREA)
  • Led Devices (AREA)
  • Glass Compositions (AREA)

Abstract

The amplifier (6) includes an optical guide (10), which is connected in series to an optical line (4), and receives the multiplex input signal coming down the line. The guide is made of a doped optical fibre or a mono-crystalline semiconductor wafer. two laser diodes (12,14) generate beams at a wavelength of 1480 nm which are injected in the guide via two multiplexers (32,34). An optical coupler (16) extracts a sample of the light amplified in the guide which is then detected by a photodiode (18). the photodiode is connected to an amplifier which controls the power supply of the two laser diodes.

Description

La présente invention concerne la régulation du gain d'un amplificateur optique à guide ou fibre dopée.The present invention relates to gain regulation an optical amplifier with a guide or doped fiber.

Le gain d'un tel amplificateur est classiquement commandé par l'intermédiaire d'un courant électrique. Ce courant alimente une source optique "de pompe" qui fournit une onde "de pompe" assurant l'excitation des éléments dopants de la fibre. Des amplificateurs connus comportent des boucles d'asservissement pour assurer une régulation de ce gain. Une telle boucle mesure la puissance d'une lumière rayonnée ou amplifiée par l'amplificateur, cette puissance étant représentative du gain et cette lumière pouvant être appelée "lumière de régulation". Puis elle compare cette puissance représentative à une puissance de consigne. Enfin, elle commande le courant d'alimentation de la source de pompe pour limiter ou annuler la différence entre ces deux puissances.The gain of such an amplifier is conventionally controlled by an electric current. This current feeds a "pump" optical source which provides a "pump" wave ensuring the excitation of the elements fiber dopants. Known amplifiers include servo loops to ensure regulation of this gain. Such a loop measures the power of a light radiated or amplified by the amplifier, this power being representative of the gain and this light can be called "regulation light". Then she compares this representative power at a setpoint power. Finally, it controls the supply current from the pump to limit or cancel the difference between these two powers.

Dans un premier amplificateur connu la lumière de régulation est une lumière d'émission spontanée rayonnée transversalement par la fibre amplificatrice. Ce premier amplificateur connu est décrit dans le document EP-A 517 503 (NTT).In a first known amplifier the light of regulation is a spontaneous radiated emission light transversely by the amplifying fiber. This first known amplifier is described in the document EP-A 517 503 (NTT).

Il présente dans beaucoup d'applications industrielles les inconvénients que la lumière de régulation doit être filtrée et que la puissance de cette lumière mesurée par un détecteur est d'une part très faible, d'autre par très sensible à de petites variations de la situation du détecteur par rapport à la fibre, ce qui perturbe la régulation.It has many industrial applications the downsides that regulating light must be filtered and that the power of this light measured by a detector is on the one hand very weak, on the other hand by very sensitive to small variations in the situation of the detector relative to the fiber, which disturbs the regulation.

Dans un deuxième amplificateur connu la lumière de régulation est une lumière d'émission spontanée qui est guidée et amplifiée par la fibre amplificatrice et qui est prélevée en sortie de celle-ci. Cette lumière s'est donc propagée dans cette fibre dans le même sens que le signal à amplifier et peut donc être qualifiée de "codirectionnelle". In a second known amplifier the light of regulation is a spontaneous emission light which is guided and amplified by the amplifying fiber and which is taken out of it. This light therefore propagated in this fiber in the same direction as the signal to amplify and can therefore be described as "co-directional".

Ce deuxième amplificateur connu est décrit dans le document EP-A-395 277 (STC-PLC). Il présente l'inconvénient que la lumière de régulation doit être soigneusement filtrée pour que la puissance mesurée ne soit pas modifiée par le fait que le détecteur reçoit une partie de la lumière du signal amplifié. Ce filtrage soigné augmente le coût et l'encombrement de l'amplificateur.This second known amplifier is described in document EP-A-395 277 (STC-PLC). It has the disadvantage that the control light must be carefully filtered so that the measured power is not modified by the causes the detector to receive some of the light from the amplified signal. This careful filtering increases the cost and the size of the amplifier.

Un but de la présente invention est d'assurer d'une manière plus simple la régulation du gain d'un amplificateur optique.An object of the present invention is to provide a simpler way the gain control of an amplifier optical.

Dans ce but cette invention a pour objet un amplificateur optique régulé comportant un guide amplificateur et une boucle d'asservissement de la puissance d'une émission spontanée amplifiée par ce guide amplificateur cet amplificateur étant caractérisé par le fait que l'émission dont la puissance est asservie est une émission contradirectionnelle et guidée par ce guide amplificateur.For this purpose this invention relates to a regulated optical amplifier comprising a guide amplifier and a power control loop amplified spontaneous emission by this guide amplifier this amplifier being characterized by the fact that the program whose power is controlled is a contradirectional emission and guided by this amplifier guide.

A l'aide des figures schématique ci-jointes, on va décrire plus particulièrement ci-après, à titre d'exemple non limitatif, comment la présente invention peut être mise en oeuvre. Lorsqu'un même élément est représenté sur plusieurs figures il y est désigné par le même signe de référence.Using the attached schematic figures, we will describe more particularly below, by way of example non-limiting, how the present invention can be implemented in action. When the same element is represented on several figures there are designated by the same sign of reference.

La figure 1 représente une vue d'une liaison optique comportant un amplificateur selon la présente invention.Figure 1 shows a view of an optical link comprising an amplifier according to the present invention.

La figure 2 représente une vue de l'amplificateur de la liaison de la figure 1.Figure 2 shows a view of the amplifier the link of figure 1.

Conformément à la figure 1 et comme connu une liaison optique comporte les éléments suivants :

  • Un ensemble émetteur 2. Cet ensemble reçoit simultanément plusieurs signaux à transmettre S1, S2, S3. Le nombre de ces signaux peut varier, le signal S3 pouvant être provisoirement absent. L'ensemble 2 émet en réponse simultanément plusieurs ondes porteuses de nature optique portant ces signaux dans plusieurs bandes spectrales distinctes. Ces bandes spectrales appartiennent à une pluralité de canaux de la liaison. Elles occupent certains de ces canaux. Une correspondance respective existe entre les signaux à transmettre, les ondes porteuses et les canaux occupés.
  • Une ligne optique 4. Cette ligne reçoit les ondes porteuses et les guide ensemble sous la forme d'un multiplex spectral.
  • Un amplificateur 6 est inseré dans la ligne 4 et amplifie le multiplex. Chaque onde porteuse reçoit ainsi une amplification. Le gain de cette amplification constitue un gain du canal correspondant à cette onde porteuse.
  • Enfin un ensemble récepteur 8 reçoit les ondes porteuses en sortie de la ligne 4. Il restitue en réponse les signaux à transmettre.
In accordance with FIG. 1 and as known, an optical link comprises the following elements:
  • A transmitter assembly 2. This assembly simultaneously receives several signals to be transmitted S1, S2, S3. The number of these signals may vary, the signal S3 possibly being temporarily absent. The assembly 2 transmits in response several carrier waves of an optical nature carrying these signals in several distinct spectral bands. These spectral bands belong to a plurality of channels of the link. They occupy some of these channels. A respective correspondence exists between the signals to be transmitted, the carrier waves and the busy channels.
  • An optical line 4. This line receives the carrier waves and guides them together in the form of a spectral multiplex.
  • An amplifier 6 is inserted in line 4 and amplifies the multiplex. Each carrier wave thus receives an amplification. The gain of this amplification constitutes a gain of the channel corresponding to this carrier wave.
  • Finally, a receiver assembly 8 receives the carrier waves at the output of the line 4. It restores in response the signals to be transmitted.

L'amplificateur 6 comporte les éléments suivants :

  • Un guide amplificateur 10 inséré en série dans la ligne 4. Ce guide est parcouru par le multiplex dans un sens direct. Il est par exemple constitué par une fibre optique dopée. Lorsqu'il est excité par une onde de pompe l'élément dopant réalise une amplification du multiplex. Il est typiquement constitué par des ions erbium Er+++. Le gain de cette amplification dépend d'une caractéristique commandée de l'onde de pompe, cette caractéristique commandée étant typiquement la puissance de cette onde. Le guide amplificateur pourrait cependant aussi être constitué par une plaquette semiconductrice monocristalline. Il serait alors inséré au sein de cette plaquette entre deux couches de confinement de types de conductivité opposés. Une action de pompe permettant une amplification du multiplex serait alors constituée par le fait qu'un courant d'alimentation injecterait des porteurs de charges dans le guide amplificateur à partir de ces deux couches de confinement.
  • Une source de pompe effectuant l'action de pompe, par exemple fournissant l'onde de pompe. Elle est typiquement dans ce cas constituée par deux diodes laser 12 et 14 émettant à une longueur d'onde de 1 480 nm et injectant deux ondes de pompe dans le guide amplificateur 10 par l'intermédiaire de deux multiplexeurs amont 32 et aval 34. Elle définit la caractéristique commandée de l'onde de pompe. Dans le cas d'un amplificateur optique semiconducteur la source de pompe serait une source électrique fournissant le courant d'alimentation de cet amplificateur.
  • Des moyens de prélèvement de régulation. Ils prélevent, à partir du guide amplificateur 10, une lumière amplifiée par ce dernier. Cette lumière sera appelée ci-après "lumière de régulation". Ces moyens de prélèvement sont typiquement constitués par un coupleur optique 16.
  • Enfin des moyens de commande de pompe 18, 20, 22. Ces moyens reçoivent la lumière de régulation. Ils commandent en réponse la source de pompe 12, 14 pour donner à la caractéristique commandée de l'onde de pompe une valeur assurant une amplification convenable du multiplex. Ils sont typiquement constitués par une diode photoréceptrice 18 pilotant un amplificateur électronique. Ce dernier fournit un courant d'alimentation aux deux diodes 12 et 14 constituant la source de pompe. Il est constitué par un préamplificateur 20 et un amplificateur de puissance 22. Ces moyens de commande de pompe réalisent ainsi un asservissement de la puissance de la lumière de régulation.
Amplifier 6 includes the following elements:
  • An amplifier guide 10 inserted in series in line 4. This guide is traversed by the multiplex in a direct direction. It is for example constituted by a doped optical fiber. When energized by a pump wave, the doping element amplifies the multiplex. It is typically made up of erbium Er +++ ions. The gain of this amplification depends on a controlled characteristic of the pump wave, this controlled characteristic being typically the power of this wave. The amplifier guide could however also consist of a monocrystalline semiconductor wafer. It would then be inserted within this wafer between two confinement layers of opposite conductivity types. A pump action allowing amplification of the multiplex would then be constituted by the fact that a supply current would inject charge carriers into the amplifier guide from these two confinement layers.
  • A pump source performing the pump action, for example providing the pump wave. It is typically in this case constituted by two laser diodes 12 and 14 emitting at a wavelength of 1480 nm and injecting two pump waves into the amplifier guide 10 by means of two upstream 32 and downstream 34 multiplexers. defines the controlled characteristic of the pump wave. In the case of a semiconductor optical amplifier, the pump source would be an electrical source supplying the supply current for this amplifier.
  • Regulatory sampling means. They take, from the amplifier guide 10, a light amplified by the latter. This light will hereinafter be called "regulation light". These sampling means are typically constituted by an optical coupler 16.
  • Finally, pump control means 18, 20, 22. These means receive the regulation light. They control in response the pump source 12, 14 to give the controlled characteristic of the pump wave a value ensuring proper amplification of the multiplex. They typically consist of a photoreceptor diode 18 driving an electronic amplifier. The latter provides a supply current to the two diodes 12 and 14 constituting the pump source. It consists of a preamplifier 20 and a power amplifier 22. These pump control means thus provide a control of the power of the regulation light.

Dans un amplificateur optique connu le prélèvement de la lumière de régulation est effectué à la sortie du guide amplificateur et cette lumière est constituée par une fraction prédéterminée du multiplex amplifié et du bruit de sortie de ce guide. Les moyens de commande d'amplificateur commandent la puissance de l'onde de pompe de manière à maintenir constante la puissance de cette lumière de régulation. La puissance totale de sortie de l'amplificateur est ainsi maintenue constante.In a known optical amplifier the sampling of the regulation light is carried out at the exit of the guide amplifier and this light is made up of a predetermined fraction of the amplified multiplex and noise output from this guide. Amplifier control means control the power of the pump wave so that keep the power of this light constant regulation. The total output power of the amplifier is thus kept constant.

Selon la présente invention les moyens de prélèvement de régulation 16 sont disposés en entrée du guide amplificateur 10. La lumière de régulation est alors constituée par une émission spontanée amplifiée dite "contradirectionnelle" parce qu'elle se propage dans un sens inverse opposé audit sens direct. Elle occupe spectralement une "bande de régulation". Un "gain de régulation" est constitué par un gain moyen présenté par le guide amplificateur dans la bande de régulation. L'asservissement de la puissance de la lumière de régulation constitue alors en même temps un asservissement du gain de régulation. Il permet ainsi d'une manière simple de réaliser un asservissement précis du gain de l'amplificateur dans diverses bandes spectrales pouvant inclure ou non la bande de régulation.According to the present invention the sampling means 16 are arranged at the input of the guide amplifier 10. The regulation light is then constituted by an amplified spontaneous emission called "contradictory" because it spreads in one direction opposite opposite to said direct sense. It occupies spectrally a "regulation band". A "gain in regulation" is constituted by an average gain presented by the guide amplifier in the regulation band. The enslavement of the power of the regulating light then constitutes at the same time a control of the gain of regulation. he thus allows in a simple way to achieve a precise control of the amplifier gain in various spectral bands which may or may not include the band of regulation.

De préférence l'amplificateur 6 comporte en outre un filtre optique 24 interposé entre les moyens de prélèvement de régulation et les moyens de commande de pompe 18, 20, 22 pour limiter la bande de régulation. Le domaine spectral occupé par les canaux de la liaison s'étend par exemple entre 1 530 et 1 560 nm, et la bande de régulation s'étend par exemple entre 1 565 et 1 570 nm environ, en longueurs d'onde. Cette bande de régulation est choisie pour que la lumière de régulation de gain soit affectée aussi peu que possible par une variation de la puissance d'entrée.Preferably the amplifier 6 further comprises a optical filter 24 interposed between the sampling means regulator and pump control means 18, 20, 22 to limit the regulation band. The spectral domain occupied by the link channels extends for example between 1,530 and 1,560 nm, and the regulation band extends for example between 1,565 and 1,570 nm approximately, in lengths wave. This regulation band is chosen so that the gain regulation light is affected as little as possible by varying the input power.

Outre les éléments déjà mentionnés l'amplificateur 6 comporte deux bornes de connexion amont 25 et aval 26, et deux isolateurs optiques amont 28 et aval 30.In addition to the elements already mentioned, the amplifier 6 has two upstream 25 and downstream 26 connection terminals, and two upstream 28 and downstream 30 optical isolators.

Claims (5)

  1. A regulated optical amplifier including an amplifier waveguide (10) and a servo-control loop (16, 24, 18, 20, 22, 12, 32, 10) for servo-controlling the power of an amplified spontaneous emission that is amplified by the amplifier waveguide, said amplifier being characterized by the fact that the emission whose power is servo-controlled is a "reverse" emission and is guided by the amplifier waveguide.
  2. An amplifier according to claim 1, said amplifier including:
    an amplifier waveguide (10) for receiving and guiding light to be amplified propagating from an input to an output of said amplifier waveguide in a forward direction, said amplifier waveguide containing amplifier elements suitable for responding to pump action by applying amplification to said light, the gain of the amplification depending on a controlled characteristic of said pump action;
    a pump source (12, 14) performing said pump action;
    regulation tapping means (16) for tapping light from said amplifier waveguide (10), which light is amplified thereby and constitutes regulation light; and
    pump drive means (18, 20, 22) receiving said regulation light and responding by driving said pump source (12, 14) so as to cause said controlled characteristic of the pump action to take a value that ensures desired amplification of said light to be amplified;
    said amplifier (6) being characterized by the fact that said regulation tapping means (16) are disposed at the input of said amplifier waveguide (10), said regulation light being constituted by an amplified spontaneous reverse emission propagating in a backward direction that is opposite from said forward direction.
  3. An amplifier according to claim 2, characterized by the fact that it further includes an optical filter (24) interposed between said regulation tapping means and said pump drive means (18, 20, 22) so as to limit a regulation band occupied in the spectrum by said regulation light.
  4. An amplifier according to claim 3, characterized by the fact that said regulation band is chosen so as to limit parasitic variations in power that occur in said regulation light in response to variations in the power of said light to be amplified received by said amplifier.
  5. An amplifier according to claim 4, in which a wavelength spectrum range occupied by said light to be amplified extends from 1,530 nm to 1.560 nm, and said regulation band extends from about 1,565 nm to about 1,570 nm, said amplifier waveguide (10) being an erbium-doped optical fiber, and said pump action being optical pumping.
EP95400052A 1994-01-13 1995-01-11 Controlled optical amplifier Expired - Lifetime EP0663708B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR9400321 1994-01-13
FR9400321A FR2714982B1 (en) 1994-01-13 1994-01-13 Regulated optical amplifier.

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Publication Number Publication Date
EP0663708A1 EP0663708A1 (en) 1995-07-19
EP0663708B1 true EP0663708B1 (en) 1999-07-21

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US (1) US5497264A (en)
EP (1) EP0663708B1 (en)
JP (1) JP3000573B2 (en)
AT (1) ATE182426T1 (en)
CA (1) CA2139814A1 (en)
DE (1) DE69510829T2 (en)
FR (1) FR2714982B1 (en)

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US11808997B1 (en) * 2022-09-19 2023-11-07 Nexus Photonics Inc. Heterogeneous photonic integrated circuits with doped waveguides

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JP2648643B2 (en) * 1991-06-03 1997-09-03 日本電信電話株式会社 Optical amplifier
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FR2714982B1 (en) 1996-02-02
JP3000573B2 (en) 2000-01-17
JPH07221712A (en) 1995-08-18
CA2139814A1 (en) 1995-07-14
ATE182426T1 (en) 1999-08-15
DE69510829D1 (en) 1999-08-26
US5497264A (en) 1996-03-05
EP0663708A1 (en) 1995-07-19
FR2714982A1 (en) 1995-07-13
DE69510829T2 (en) 1999-12-09

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